Stack-up your chances when co-extruding. Formulations and rheology are a moving target. An analytical approach means offering more than just extrusion equipment.
Our multi-layer extrusion technology is intimately tied to our understanding of material flow. We study it, simulate it, build prototypes to validate it, work with material suppliers to control it, and run customer trials at our center to industrialize it. With this better understanding, we go beyond simply offering extrusion systems. We offer answers to today’s needs.
Taking a close look inside
Polymer formulations change, just as the end-user markets demand change. The trends are present in Automotive, Heating & Plumbing, Fiber Micro-ducts, and more. Multi-layer pipe & tube constructions are constantly revisited. Automotive tubes for cooling of electrical batteries are one clear example. For each polymer employed on our lines, Maillefer’s R&D team takes a close look at what goes on inside the extrusion process, particularly at the exit point, inside the extrusion head itself.
Concentrated on the weld
The all-familiar coat hanger distributor has a straightforward design that pushes the melt flow towards a single weld line. The line is an indication of where the two opposing flows join as they wrap around to form a product (e.g. cable or pipe). It is here that we find a weaker cohesion of polymer molecules. Concentricity, layer thickness, and even overall product quality are potentially compromised. The weld effect is possible in any of the layers and can be compounded by their number, though for some materials it is less notable than for others.
Helical design
A well-known way to overcome the problem of a weld line is to use helical type distributors that deconstruct the flow into several channels. In the design process, the shape, depth, and angle of the helical channels are optimized to obtain an even distribution. The number of helical channels can vary from two(2) to eight(8). The material flows both along the channels and transversely from one channel to another, creating gradual transitions between the flow lines.
FEA prediction
When calculated with Finite Element Analysis (FEA) models or when made visible with color masterbatch, the resulting weld line shows a characteristic spiral shape. This helps to get a high quality extruded product.
The chart above shows the simulation of an eight (8) channel helical distributor, optimized for the use of three different polymers. Only one quarter of the circular distribution is displayed for better clarity.
The simulation above highlights an eight channels helical distributor. Particular attention was paid to the shear level at the steel-polymer interface. Too little shear can lead to stagnation and scorch build-up.
Understanding the fundamentals
Despite its advantages, the helical distributor is not always the primary choice. Other criteria may come into play, such as residence time (for heat-sensitive materials), wall shear rate, or pressure drop. The combination of spiral and coat hanger feeders is possible in a multilayer extrusion head. Such a mix takes full account of the rheological properties of the coextruded materials. We employ simulation tools and a fundamental understanding to optimize the distributor designs. The results are then confirmed during our R&D tests and with key material suppliers.
Stacking up your chances
Mastering multi-layer extrusion can be likened to the old saying “The more you know, the more you realize you don’t know”. Stack-up your chances when co-extruding multiple layers. Formulations and rheology are a moving target. For material savings, tight tolerances, and low scrap, you can rely on a partner that has the best tools to determine ideal flow behavior. Maillefer’s distributor designs let you best respond to the specific application.
Norbert Jotterand Yves Zweifel, PhD
Components & Heads Manager R&D Manager
norbert.jotterand@maillefer.net yves.zweifel@maillefer.net